It is well known that when two or more regular patterns such as parallel strips are overlaid at a random orientation that a resultant larger regular pattern is formed called the Moire fringe. This pattern is formed due to the periodic summation of periodic patterns which will vary from place to place if the two patterns are slightly displaced from identical overlap or are of slightly different geometry. If the patterns move with respect to one another the resultant Moire fringe will be seen to move at a faster rate than the relative pattern motion.
The Moire fringe effect has been used to make possible displacement measurements with instruments of limited resolution by virtue of the size and speed magnification effects. These systems use either systems of grating transparencies or slotted plates for the reference patterns. Since there are limitations to the physical characteristics and definition of such devices, a purely optical system can be of advantage. With the advent of inexpensive laser diodes, a practical source of monochromatic light is available.